Hydromechanical transmissions, including hydromechanical powersplit transmissions, are used in hydraulic hybrid vehicles. Such vehicles may include a vehicle prime mover such as an internal combustion engine, at least one hydraulic pump motor unit, a gear set such as a planetary gear set, and an output shaft connecting the planetary gear set to a drive shaft of the vehicle. The internal combustion engine and the hydraulic pump motor unit are connected to the gear set, and the gear set splits power from the internal combustion engine and from the hydraulic pump motor unit in a motoring mode to rotate the drive shaft and propel the vehicle. The pump motor unit may also be used in a pumping mode to capture energy under certain conditions such as braking the vehicle, and the captured energy may be stored in an energy storage device such as a hydraulic accumulator to power the hydraulic pump motor unit in the motoring mode.
Hydraulic circuits, including hydraulic circuits used in hydromechanical transmissions and elsewhere, usually have some amount of gas dissolved in or entrained in the hydraulic fluid. This gas may be, for example, air from the atmosphere or nitrogen that permeates through the bladder of a known nitrogen bladder type accumulator and into the hydraulic fluid.
Gas that is dissolved in or entrained in hydraulic fluid in a hydraulic circuit can cause various known problems, including noise, reduced component response due to spongy behavior of aerated fluids, cavitation damage and fluid degradation. The present invention addresses this technical problem and provides a de-aerating hydraulic circuit and method that removes gas from the hydraulic fluid of the circuit. The circuit and method according to the present invention also charges a low pressure side of the circuit at start up. Further, the circuit and method according the present invention then de-aerates the hydraulic fluid by flow from the high pressure side to the low pressure side and from the low pressure side to the atmospheric or low pressure reservoir.